Often in display devices, it is useful to incorporate electronic components onto the glass substrate used in the display device. This is the case in liquid crystal display (LCD) devices. In LCD devices, a layer of liquid crystal material is modulated by voltages, which are generated by transistor arrays. Often, the transistors are thin-film transistors (TFT), and are complementary metal oxide semiconductor (CMOS) devices.
The LCD displays often comprise a glass substrate with the transistors formed over the glass substrate, and beneath a layer of LC material. The transistors are arranged in a patterned array, and are driven by peripheral circuitry to provide to switch on desired voltages to orient the molecules of the LC material in the desired manner.
The transistors of the display are often formed from a semiconductor material disposed directly on or over the glass substrate. Because the mobility the carriers of a semiconductor is generally greater in polycrystalline materials compared to amorphous materials, it is beneficial to grow polycrystalline semiconductor layers on or over the glass substrate of the LCD display. For example, higher mobility carriers enable faster transistors for video applications. Alternatively, the higher mobility enables reduced feature-size transistors, which facilitates creation of higher aspect ratio displays.
In addition to the referenced benefits of fabricating polycrystalline materials and devices on a glass display surface, the creation of polycrystalline (e.g. polysilicon or ‘poly’) transistors on the glass display surface enables the creation of driver circuitry directly on the display, thereby eliminating the need for Tape Automated Bonding (TAB) connections and their attendant poor reliability. Moreover, if the display is based on organic light emitting devices (OLED'S), it is beneficial to incorporate poly transistors to supply the relatively high current requirements of the OLED's.
A significant drawback to growth of poly on glass substrates is the comparatively high temperatures the growth sequences require. While strides have been made to reduce the growth temperature of poly, it is often necessary to pre-anneal the glass substrate to thermally stabilize the glass for subsequent poly processing. This thermal processing, as well as other thermal processing can alter the physical structure of the glass. For example, the glass may expand or contract as a result of the processing.
One measure of the alteration of the glass by thermal processing is known as glass strain, which is known as compaction when negative and expansion when positive. The glass strain is proportional to the change in fictive temperature, which is the temperature of the glass when the molecular structure reaches a certain state of order. At high glass temperatures the fictive temperature equals the ordinary glass temperature because the glass is able to equilibrate very quickly with its ambient temperature. As the temperature is reduced, the glass viscosity rises exponentially with falling temperature and the speed of glass equilibration is dramatically reduced.
Thus, as the temperature is reduced, the glass “falls out of equilibrium” because of its inability to maintain equilibrium as the temperature changes. In this case the fictive temperature lags thermal temperature, and ultimately the fictive temperature ‘stalls’ at some higher temperature at which the glass no longer could equilibrate quickly enough to keep up with its cooling rate. The final fictive temperature will depend on how quickly the glass was cooled, and will typically be in the range of approximately 600° C. to approximately 800° C. for LCD substrate glass at room temperature.
As can be appreciated, therefore, the fictive temperature depends on the thermal history of the glass. As such, the compaction and expansion depends on the thermal history of the glass. The strain, which is proportional to the difference in the distance between two marks on a glass substrate before and after a heat treatment cycle divided by the distance before, must remain within a specified value that is set by the user. For example, in LCD display systems, the display substrate is normally required to have a glass strain having an absolute value of approximately 10 ppm, often a compaction level between approximately −10 ppm and approximately 0 ppm. Compaction values with magnitude greater than approximately 10 ppm can result in misalignment and mis-registration of overlapping patterns in an LCD display, resulting in image distortion, for example.
A significant problem remains to curb the glass strain in a glass substrate throughout its processing to the final product that implements the glass. For example, as referenced above, in applications where it is necessary to grow polycrystalline semiconductor materials on the glass using thermal treatments, the resultant compaction may be too great.
What is needed therefore is a method and apparatus that addresses the drawbacks of glass strain caused by thermal cycling in processing the glass.